1. Field of the Invention
This invention relates to an electrostatic capacitance type transducer and a method for producing the same, which is used for a pressure sensor, an acceleration sensor, and so on.
2. Description of the Related Art
Electrostatic capacitance type transducer An electrostatic capacitance type transducer has conventionally used for a measuring instrument, such as a pressure sensor and an acceleration sensor. The electrostatic capacitance type transducer has a structure that a movable electrode and a substrate, having a film-shaped fixed electrode facing toward the movable electrode, are opposed to each other to make a space between them, in which the displacement of the movable electrode with relation to the substrate is detected as a change of electric capacitance between the movable electrode and the fixed electrode.
For example, in an electrostatic capacitance type pressure sensor, a diaphragm as the movable electrode, made of silicon having conductivity, is placed to oppose to the substrate through the space, in which the substrate has the fixed electrode on a detecting face facing toward the diaphragm. When pressure of fluid is measured, the fluid measured is guided onto a face of the diaphragm which does not face toward the substrate. The pressure of the fluid is changed into an electric signal by detecting the displacement of the diaphragm, caused by the pressure of the fluid, as a change of electrostatic capacitance.
For the precise measurement by using the electrostatic capacitance type transducer such as the pressure sensor as described above, insulation between the movable electrode and the fixed electrode is required. But, when the transducer is in a humid atmosphere, water in the ambient air touches the electrode or the substrate and insulation resistance can be decreased. As insurance against the insulation resistance, an insulating skin is formed on the surface of the fixed electrode or the movable electrode by partially coating resin, glass or the like in order that water does not directly touch the fixed electrode or the movable electrode.
In producing the electrostatic capacitance type transducer, a voltage of approximately 400 V is applied to the glass substrate and the silicon movable electrode under high temperatures of approximately 400.degree. C. to connect the substrate to the movable electrode directly, in other words, by anodic bonding.
Anodic-Bonding
However, the connecting strength when the substrate and the movable electrode are mutually connected by means of anodic bonding is inferior, and it leads to decrease of yield rate.
More specifically, cleanliness of both faces of the members which are mutually connected is a big factor for the connecting strength in the anodic bond. But, the fixed electrode of the electrostatic capacitance type transducer is made of metal material that differs from materials, such as glass, of the substrate, hence constraints of a cleaning fluid and a cleaning manner as compared with the use of the same material. Moreover, cleaning is complicated by through-holes formed on the substrate for receiving an output from the fixed electrode, or the like. Therefore the connecting faces of the substrate and the movable electrode are not sufficiently cleaned.
It is difficult to clean organic matter at the molecular level which midbonds to the substrate even with an organic solvent. A thermal strong sulfuric acid or an alkali solution stronger than an organic solvent cannot be used of the fixed electrode and the substrate for reasons of damage. A clean by using dry-etching, ozone or plasma facilitates adsorption of water molecular onto the surface of the substrate, thus decreasing insulation resistance of the substrate.
Accordingly, trace contaminants of the organic matter on the surface of the substrate connected are not completely cleaned. There is frequently decrease of contact yield rate which is suspected to be traceable to the contaminants on the connecting face.
Concerning a semiconductor pressure sensor, a method, in which a ceramic insulation film as a top coat is formed on a face of a diaphragm, facing toward a substrate, and the diaphragm and the substrate are mutually connected through the insulation film by means of the anodic bond, is proposed (Japanese Patent laid-open No. Sho63-110670).
It is considered with the application of the above method to the electrostatic capacitance type transducer that the decrease of insulation resistance between the electrodes can be avoided by forming an insulation film on the movable electrode and connecting the movable electrode and the substrate through the insulation film by means of the anodic bond. But, in the above method, although insulation between the movable electrode and the fixed electrode is retained, insulation resistance between the fixed electrodes cannot be avoided when the plural fixed electrodes are formed on the substrate. For the aforementioned reasons, the cleanliness of the substrate cannot be enhanced, so that the sufficient connecting strength cannot be obtained. Therefore, the decrease of contact yield rate remains.
In order to partially form the insulating skin on the surface of the substrate for enhancing the insulation, the insulation film should undergo partial patterning through photolithography or the like, resulting in the complicated processes for producing. Furthermore, in many cases, the substrate and the insulating skin are a glass substance to be formed as an insulator. The glass substance dissolves in an etching fluid, resulting in the confused determination of the end of etching.
Miniaturization
The conventional electrostatic capacitance type transducer is relatively large in size, so that abnormal skew of the movable electrode presents no disadvantage even in anodic bonding since a spaced range between the movable electrode and the substrate is large. However, the spaced range between the movable electrode and the substrate is smaller by miniaturizing the pressure sensor or the like. Therefore, strong electrostatic attraction is created between the movable electrode and the fixed electrode when a high voltage is applied during anodic bonding. The movable electrode is drawn toward the substrate and skewed or touched to the substrate. Where anodic bonding is continued during the above state, the movable is formed not in an even state but in a skewed state toward the substrate. And, harmful stress is given to the movable electrode, with the result that the movable electrode does not have the displacement in response to the pressure or the like.
The U.S. Pat. No. 4,384,899 and Japanese Patent Laid-open No. Hei2-290524 are known as technology for resolving the disadvantages described thus far. In the above methods, a high voltage is applied to the fixed electrode formed on the substrate to equalize a potential of the fixed electrode to a potential of the movable electrode, thereby preventing generation of electrostatic attraction.
Some pressure sensors have the plural fixed electrodes on the substrate, such as a central electrode positioned the center and a peripheral electrode surrounding the central electrode. In the above pressure sensor, the pressure can be further accurately detected by measuring a difference of electrostatic capacitance on each fixed electrode.
The above pressure sensor has been deliberated about the prevention of the touch or abnormal skew of the movable electrode, as explained above, by equalizing potentials of the central electrode and the peripheral electrode to a potential of the movable electrode. For this requirement, a voltage can be applied to the central electrode and the peripheral electrode by using signal receiving portions, formed on the surface (the reverse of the surface formed with the central electrode and the peripheral electrode) of the substrate, to be extended from each electrode.
With the miniaturization of the pressure sensor, the size of the signal receiving portion is extremely smaller, and further, a space between each signal receiving portion and an electrode for anodic bonding, surrounding the signal receiving portions, is shorter. Therefore, it is difficult that a terminal for voltage application is directly connected to each signal receiving portion as disclosed in the U.S. Pat. No. 4,384,899. Therefore, a leading portion extended from each signal receiving portion is formed outside the anodic bonding electrode, and the application should be carried out by connecting a conductor wire to the leading portion.
However, as shown in FIG. 9, in a chip-shaped pressure sensor 200, in order to extend each leading portion 204 of signal receiving portions 202 and 203 outside an anodic bonding electrode 201, each leading portion 204 is needed to be pass through a discontinuous portion 201A of the anodic bonding electrode 201. Where the number of the leading portions 204 is two for the central electrode and the peripheral electrode, the length L of the discontinuous portion 201A is longer, thus the inferior connecting strength between the movable electrode and the substrate. And further, for example, where more than three signal receiving portions and the leading portions for each signal receiving portion are formed, the implementation of the anodic bonding itself is difficult.
On the other hand, when the plural leading portions are passed through the short discontinuous portion, the leading portion approaches the anodic bonding electrode, thereby allowing a disadvantage for a withstand voltage against the high voltage applied during anodic bonding.
As a result, when the plural fixed electrodes are formed on the substrate, the leading portion approaches the anodic bonding electrode, thereby allowing a disadvantage for a withstand voltage against a high voltage applied during anodic bonding.
As a result, when the plural fixed electrodes are formed on the substrate made of glass or the like, there is a limitation of increasing the connecting strength between the substrate and the movable electrode while maintaining the sufficient withstand voltage.